The increased use of computers and other sensitive electric devices has resulted in the proliferation of electrical loads that cannot tolerate power quality problems such as voltage sags and swells, momentary interruptions, and frequency excursions. Computer processing and data centers are not the only facilities susceptible to these power quality problems, manufacturers, laboratories and process industries are affected as well.
Voltage sags and momentary interruptions are the most commonly encountered power quality problem. An uninterruptible power supply (UPS) has the ability to protect sensitive loads from these power quality problems. For longer term power quality problems, such as outages exceeding 30 to 60 seconds in duration, back-up generator systems may be integrated with a UPS system. The UPS system responds immediately to the power quality problem and provides the time necessary to bring a back-up generator system on-line to provide longer term supplemental power.
The UPS system should seamlessly integrate with the utility power source and operate transparently to the sensitive load equipment. In a typical UPS operation, the utility source may be in a voltage sagged condition for some period before the sag is detected and the UPS output ramps up to normal output. During this period, the flux in downstream transformers may develop a direct current (DC) offset. The DC offset of the flux generally drives the downstream transformers into saturation during the first loop (and to a lesser degree, subsequent loops) of a polarity opposite that of the loop in which the sag occurred. The magnetizing current requirement during the saturation period is quite large, and presents a challenge for the UPS.